- Any place where you want to move your site to a new domain but can't control server headers. This can be really useful to help avoid lock in on free hosts for example.
- Companies that register several domain names as landing pages for offline advertising, a practice often used for tracking performance of the campaign, can now keep users on the original domain but keep spiders going to the right place so Google doesn't end up with 10 copies of the site.
- Syndicating content - if your syndication partner agrees - you can now make sure that your "link juice" flows back to the original article in Google.
I think we'll see lots of creative uses of this tweak in the future as we've seen with the original rel=canonical. Let's just hope those creative talents are used for the power of good.